Exclusive J /ψ photoproduction off protons in ultraperipheral p -Pb collisions at √sNN =5.02TeV

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Exclusive J=ψ Photoproduction off Protons in Ultraperipheral p-Pb Collisions at ffiffiffiffiffiffiffi s

NN

p ¼ 5 . 02 TeV

B. Abelevet al.^* (ALICE Collaboration)

(Received 1 July 2014; published 5 December 2014)

We present the first measurement at the LHC of exclusive J=ψ photoproduction off protons, in ultraperipheral proton-lead collisions at pffiffiffiffiffiffiffiffisNN¼5.02TeV. Events are selected with a dimuon pair produced either in the rapidity interval, in the laboratory frame,2.5< y <4(p-Pb) or−3.6< y <−2.6 (Pb-p), and no other particles observed in the ALICE acceptance. The measured cross sectionsσðγþp→ J=ψþpÞ are 33.22.2ðstatÞ 3.2ðsystÞ 0.7ðtheorÞ nb in p-Pb and 28436ðstatÞ^þ27₋₃₂ðsystÞ 26ðtheorÞnb in Pb-pcollisions. We measure this process up to about 700 GeV in theγpcenter of mass, which is a factor of two larger than the highest energy studied at HERA. The data are consistent with a power law dependence of theJ=ψ photoproduction cross section inγpenergies from about 20 to 700 GeV, or equivalently, from Bjorken x scaling variable between ∼2×10⁻² and ∼2×10⁻⁵, thus indicating no significant change in the gluon density behavior of the proton between HERA and LHC energies.

DOI:10.1103/PhysRevLett.113.232504 PACS numbers: 25.20.Lj, 13.40.-f, 14.40.Pq, 25.75.Cj

Exclusive J=ψ photoproduction off protons is defined by a reaction in which the J=ψ is produced from a γp interaction, where the proton emerges intact: γþp→ J=ψþp. This process allows a detailed study of the gluon distribution in the proton, since its cross section is expected to scale as the square of the gluon probability density function (PDF), according to leading order QCD calculations[1]. The mass of the charm quark provides an energy scale large enough to allow perturbative QCD calculations, albeit with some theoretical uncertainties[2]. This process provides a powerful tool to search for gluon saturation [3,4], which is the most straightforward mechanism to slow down the growth of the PDF for gluons carrying a small fraction of the momentum of hadrons (Bjorken xscaling variable). Finding evidence of gluon saturation has become a central task for present experiments and for future projects[5,6]that aim to study quantum chromodynamics (QCD).

Both ZEUS and H1 Collaborations measured the exclu- siveJ=ψ photoproduction off protons atγpcenter-of-mass energies ranging from 20 to 305 GeV[7–9]. This process has also been studied in pp[10], pp¯[11], and heavy-ion collisions[12–14].

In this Letter we present the first measurement of exclusive J=ψ photoproduction in collisions of protons with Pb nuclei at center-of-mass energy per nucleon pairffiffiffiffiffiffiffiffi

sNN

p ¼5.02TeV. TheJ=ψis produced by the interaction

of a photon with either a proton or a nuclear target, where the photon is emitted from one of the two colliding particles. Although bothγþp→J=ψþpandγþPb→ J=ψþPb can occur, the Pb electric charge makes photon emission from the ion to be strongly enhanced with respect to that from the proton[15,16].

The main ALICE detector used in this analysis is the single-arm muon spectrometer[17], covering the pseudorapidity interval−4.0<η<−2.5. The beam directions of the LHC were reversed in order to measure both forward and backward rapidity. Thus,J=ψs are reconstructed in the 2.5< y <4.0(p-Pb) and−3.6< y <−2.6(Pb-p) rapidity intervals, whereyis measured in the laboratory frame with respect to the proton beam direction. (The ALICE detector acceptance is given in the laboratory pseudorapidityη. The convention in ALICE is that the muon spectrometer is located atη<0. In contrast, the laboratory rapidityywill change sign according to the proton beam direction, from which it takes its orientation. In p-Pb, for example, the proton goes in the η<0 direction, and y >0.) The γp center-of-mass energy Wγp is determined by the J=ψ rapidity: W²γp¼2E_pMJ=ψexpð−yÞ, where MJ=ψ is the J=ψ mass, y is the J=ψ rapidity, and Ep is the proton energy (Ep¼4TeV in the lab frame), while the Bjorken x scaling variable is given by x¼ ðM_J=ψ=W_γpÞ². We study 21< W_γp<45GeV for y >0 and 577< W_γp<

952GeV for y <0, thereby exceeding the W_γp range of HERA.

The muon spectrometer consists of a ten interaction length absorber, followed by five tracking stations, each made of two planes of cathode pad chambers, with the third station placed inside a dipole magnet with a3 T · m integrated magnetic field. The muon trigger system,

* Full author list given at the end of the article.

Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published articles title, journal citation, and DOI.

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downstream of the tracking chambers, consists of four planes of resistive plate chambers placed behind a 7.2 interaction length iron wall. The single muon trigger threshold for the data used in this analysis was set to transverse momentum pT ¼0.5GeV=c. Other detectors used in this analysis are the silicon pixel detector (SPD), VZERO, and zero degree calorimeters (ZDCs) [17]. The central regionjηj<1.4is covered by the SPD consisting of two cylindrical layers of silicon pixels. The pseudorapidity interval 2.8<η<5.1 is covered by VZERO-A and

−3.7<η<−1.7by VZERO-C. These detectors are scin- tillator tile arrays with a time resolution better than 1 ns, allowing us to distinguish between beam-beam and beam- gas interactions. The two ZDCs are located at 112.5m from the interaction point, and are used to detect neutrons and protons emitted in the very forward region.

The trigger for the p-Pb configuration required two oppositely charged tracks in the muon spectrometer, and a veto on VZERO-A beam-beam interactions. In the Pb-p configuration, the trigger purity was improved with respect to the p-Pb by suppressing beam-induced backgrounds. This was achieved by requiring at least one hit in the VZERO-C beam-beam trigger and a veto on the VZERO-A beam-gas trigger. The integrated luminosity L was corrected for the probability that exclusivity requirements could be spoiled by multiple interactions in the same bunch crossing. This pile-up correction is on average 5%, giving L¼3.9nb⁻¹3.7%ðsystÞ for p-Pb andL¼4.5nb⁻¹3.4%ðsystÞ for Pb-p data[18].

Events with exactly two reconstructed tracks in the muon spectrometer were selected off-line. The muon tracks had to fulfill the requirements on the radial coordinate of the track at the end of the absorber and on the extrapolation to the nominal vertex, as described in Refs. [12,19]. Both track pseudorapidities were required to be within the chosen range

−4.0<ηtrack<−2.5forp-Pb and−3.7<ηtrack<−2.5for Pb-p. Track segments in the tracking chambers must be matched with corresponding segments in the trigger chambers. The dimuon rapidity was in the range2.5< y <4.0for p-Pb and−3.6< y <−2.6for Pb-p. The chosen range in Pb-pensured that the muon tracks are in the overlap of the muon spectrometer and VZERO-C geometrical acceptance, as VZERO-C was part of the trigger in Pb-p. A cut on VZERO timing was imposed off-line to be compatible with crossing beams. In order to reduce contamination from nonexclusiveJ=ψs that come mainly from proton dissociation, only events with no midrapidity tracklets (track segments formed by two hits at each SPD layer) were kept.

For the same reasons, events with neutron or proton activity in any of the ZDCs were rejected.

The dimuon invariant mass spectra (M_μ^þ_μ⁻) after these selections are shown in Fig. 1. The J=ψ peak is clearly visible in both data sets, and is well described by a Crystal Ball parametrization[20], which yields masses and widths in agreement with the Monte Carlo simulations. The

dimuon continuum is well described by an exponential as expected from two-photon production of continuum pairs (γγ →μ^þμ⁻) [12,13].

The extracted number of J=ψs obtained from the invariant mass fit includes a mix of exclusive and nonexclusive J=ψ candidates. A different pT distribution is expected from exclusive and nonexclusiveJ=ψ events[9].

For this reason, the number of exclusive J=ψs can be determined from the dimuon p_T distributions shown in Fig.2. The bulk of dimuon events havingp_T <1GeV=cis mainly due to exclusive J=ψ production, while the tail extending up to higherpT on the top panel (p-Pb) comes from nonexclusive interactions. Exclusive J=ψ coming fromγpinteractions andγγ contribute to bothpT spectra.

In addition, for p-Pb, a background, coming from nonexclusive J=ψs and nonexclusive γγ→μ^þμ⁻ events was taken into account, while for the Pb-psample a contribution from coherentJ=ψ inγPb interactions was considered.

The latter process was neglected inp-Pb as it amounts to less than 2% [16]. If modifications to the nuclear gluon distribution, also known as nuclear shadowing, are considered, this contribution would be even smaller. Here, an additional 50% reduction is expected[13]from shadowing effects. ThepT shapes for theJ=ψ inγp,γγ→μ^þμ⁻, and coherentJ=ψ inγPb components (Monte Carlo templates) were obtained usingSTARLIGHT[21,22]events folded with the detector response simulation. Forp-Pb, these templates were fitted to the data leaving the normalization free for

)2Dimuon candidates / (50 MeV/c

20 40 60 80 100 120 140 160 180 200

0 . 4

<

y

<

5 . 2 E

C I L A

= 5.02 TeV sNN

p-Pb

2 ) ( GeV/c

μ- μ+

M

1.5 2 2.5 3 3.5 4 4.5 5

0 5 10 15 20 25 30

6 . 2 -

<

y

<

6 . 3 - E

C I L A

= 5.02 TeV sNN

Pb-p

FIG. 1 (color online). Invariant mass distribution for events with two oppositely charged muons, for both forward (top panel) and backward (bottom panel) dimuon rapidity samples.

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J=ψ in γp and the nonexclusive background. The γγ→ μ^þμ⁻ component was constrained from the invariant mass fit shown in Fig. 1 [12]. The nonexclusive contributions were subtracted using this fitting procedure, givingNJ=ψ. ThepTdistribution of nonexclusiveJ=ψ candidates and the nonexclusive dimuon continuum were obtained from data, using the same event selection as above, but requiring events to have more than two hits in the VZERO-C counters.

At HERA the ratio of the nonexclusive J=ψ production cross section to the exclusive one was found to decrease withWγp[9]. Extrapolating, this means a factor 2 smaller nonexclusiveJ=ψcontribution in the Pb-psample. We note that for this sample dissociation products went towards the VZERO-A counter, which was used as a veto at trigger level, providing an explanation on the negligible nonexclusive contribution observed.

The number of exclusiveJ=ψcoming fromγpinteractions (N^exc_J=ψ) was obtained asN^exc_J=ψ¼N_J=ψ=ð1þf_DÞ, wheref_Dis the fraction ofJ=ψmesons coming from the decay ofψð2SÞ.

Following the procedure described in Refs. [12,13], we obtained fD¼7.9^þ2.4_−1.9% (syst) in p-Pb and fD¼11^þ3.6_−2.8% (syst) in Pb-p. The contribution of exclusiveχ_c states was neglected, as these are expected to be strongly suppressed in proton-nucleus collisions [23,24]. The resulting yield is N^exc_J=ψ ðp-PbÞ ¼41428ðstatÞ 27ðsystÞ.

N^exc_J=ψin the Pb-psample was obtained by event counting, and then subtracting theγγand theγPb components as well as the feed-down fromψð2SÞdecays. Based on our recent

coherentJ=ψ results in γPb[12], taking into account the difference in the center-of-mass energy, we estimated that 72(stat) events are expected in this sample. We obtained N^exc_J=ψ ðPb-pÞ ¼719ðstatÞ^þ2₋₅ðsystÞ. A compatible number forN^exc_J=ψ was found studying theJ=ψ pT (see Fig. 2 bottom panel). The exclusiveJ=ψ template was obtained by changing the exponential slope of thep²_T spectrum in

STARLIGHTfrom its default value of 4.0 to6.7ðGeV=cÞ⁻². This value agrees with an extrapolation of the Wγp

dependence of thep²_T slope seen by H1[9].

The product of the detector acceptance and efficiency A ×εforJ=ψ was calculated usingSTARLIGHTand ranges from 11% to 31% for the rapidity intervals corresponding to the measurements given in Table II. The systematic uncertainties on the measurement of theJ=ψ cross section are listed in Table I. The cross section corresponding to exclusive J=ψ photoproduction off protons was obtained using ðdσ=dyÞ ¼ ððN^exc_J=ψÞ=ðA ×εÞ× BR ×L×ΔyÞ, where BR is the branching ratio andΔy is the rapidity interval.

We obtainedðdσ=dyÞ ¼6.420.43ðstatÞ 0.61ðsystÞμb for p-Pb and ðdσ=dyÞ ¼2.460.31ðstatÞ^þ0.24_−0.28ðsystÞμb for Pb-pcollisions (see TableII).

We measured the cross section for the exclusive γγ→μ^þμ⁻ process at invariant mass 1.5< Mμ^þμ⁻ <

2.5GeV=c²and in the rapidity range2.5< y <4.0, using the same technique as for the J=ψ to remove the nonexclusive background, obtaining σðγγ→μ^þμ⁻Þ ¼1.76 0.12ðstatÞ 0.16ðsystÞ μb for this kinematic range. The

STARLIGHT prediction for this standard QED process is 1.8μb, which is in good agreement with this measurement.

This provides an additional indication that the nonexclusive background subtraction is under control.

The cross section ðdσ=dyÞðpþPb→pþPbþJ=ψÞ is related to the photon-proton cross section, σðγþp→ J=ψþpÞ≡σðW_γpÞ, through the photon flux,dn=dk:

Dimuon candidates / (100 MeV/c)

20 40 60 80 100

= 5.02 TeV sNN

ALICE p-Pb 2.5<y<4.0

<3.3 GeV/c2 μ-

μ+

2.8<M

(GeV/c) Dimuon pT

0 0.5 1 1.5 2 2.5 3 3.5 4

0 5 10 15 20 25

Sum ψ

Exclusive J/

Non-exclusive background μ-

μ+

γ γ γ+Pb

= 5.02 TeV sNN

ALICE Pb-p -3.6<y<-2.6

<3.3 GeV/c2 μ-

μ+

2.8<M

FIG. 2 (color online). Transverse momentum distribution for events with two oppositely charged muons, for both forward (top panel) and backward (bottom panel) dimuon rapidity samples.

TABLE I. Summary of the contributions to the systematic uncertainty for the integratedJ=ψcross section measurement for the full rapidity interval.

Source p-Pb Pb-p

Signal extraction 6% ^þ0.0_−6.0%

Luminosity[18] 3.3% 3.0%

Tracking efficiency[19] 4% 6%

Muon trigger efficiency[19] 2.8% 3.2%

Matching 1% 1%

VZERO-C efficiency 3.5%

Total uncorrelated 8.5% _−10.2^þ8.3%

Luminosity[18] 1.6% 1.6%

Branching ratio[25] 1% 1%

VZERO-A veto efficiency ^þ2₋₀^._.₀⁰% ^þ2₋₀^..0⁰% Feed-down ⁻²_þ1^._.²₈% ^þ2₋₃^..1⁶%

J=ψ acceptance 3% 3%

Total 9.6% _−11.3^þ9.6%

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dσ

dyðpþPb→pþPbþJ=ψÞ ¼kdn

dkσðγþp→J=ψþpÞ:

Here,kis the photon energy, which is determined by the J=ψ mass and rapidity, k¼ ð1=2ÞMJ=ψexpð−yÞ. The average photon flux values for the different rapidity intervals were calculated using STARLIGHT and are listed in TableII. ThehW_γpiis calculated by weighting with the product of the photon spectrum and the cross sectionσðγpÞ from STARLIGHT. The photon spectrum is calculated in impact parameter space requiring that there should be no hadronic interaction. The uncertainty in this approach is estimated by increasing or decreasing the Pb radius with 0.5fm, corresponding to the nuclear skin thickness and is of the same order as the upper limit for the difference between the proton and neutron radius of Pb when calculating the hadronic interaction probability. This gives an uncertainty of 9% in the photon flux for the high energy data point and 2% at low energy (see Table II). The uncertainty is larger for the high photon energies since here one is dominated by small impact parameters and thus more sensitive to the rejection of hadronic interactions with impact parameters near the Pb radius.

Figure3 shows the ALICE measurements for σðWγpÞ. Comparisons to previous measurements and to different theoretical models are also shown. As mentioned earlier,

σðWγpÞis proportional to the square of the gluon PDF of the proton[1]. For HERA energies, the gluon distribution at the low Bjorkenxscaling variable is well described by a power law in x [26], which implies the cross section σðW_γpÞ will also follow a power law. A deviation from such a trend in the measured cross section asxdecreases, or equivalently, asWγp increases, could indicate a change in the evolution of the gluon density function, as expected at the onset of saturation.

Both the ZEUS and H1 Collaborations[7–9]fitted their data using a power law σ∼W^δγp, obtaining δ¼0.69 0.02ðstatÞ 0.03ðsystÞ, and δ¼0.670.03ðstatþsystÞ, respectively. Because of the large HERA statistics, a simultaneous fit of H1, ZEUS, ALICE low energy points data gives power-law fit parameters almost identical to those obtained from HERA alone. A fit to ALICE data alone givesδ¼0.680.06ðstatþsystÞ, only uncorrelated systematic errors were considered here. Thus, no deviation from a power law is observed up to about 700 GeV.

Two calculations are available from the JMRT group [27]: the first one referred to as LO is based on a power law description of the process, while the second model is labeled as NLO, and includes contributions which mimic effects expected from the dominant NLO corrections.

Because both JMRT models have been fitted to the same data, the resulting energy dependences are very similar. Our data support their extracted gluon distribution up to x∼2×10⁻⁵. The STARLIGHT parameterization is based on a power law fit using only fixed-target and HERA data, giving δ¼0.650.02. Figure 3 also shows predictions from the b-SAT eikonalized model [28] which uses the color glass condensate approach[29] to incorporate saturation, constraining it to HERA data alone. The results from the models mentioned above are within one sigma of our measurement. The b-SAT 1-Pomeron prediction taken from Ref.[5]also agrees with the ALICE low energy data points, but it is about 4 sigmas above our measurement at the highest energy.

LHCb recently published results for σðW_γpÞ based on exclusive J=ψ production in pp collisions [10]. Their analysis, using data from a symmetric system, suffers from the intrinsic impossibility of identifying the photon emitter and the photon target. Since the nonexclusive background, as mentioned above, depends onW_γp, this feeds into the uncertainty in the subtraction of these processes, making TABLE II.ffiffiffiffiffiffiffiffisNN Differential cross sections for exclusiveJ=ψ photoproduction off protons in ultraperipheralp-Pb and Pb-pcollisions at p ¼5.02TeV. The correspondingJ=ψ photoproduction cross sections in bins ofWγp are also presented.

Rapidity ðdσ=dyÞðμbÞ kðdn=dkÞ Wγp (GeV) hWγpi(GeV) σðγþp→J=ψþpÞðnbÞ 2.5< y <4.0 6.420.43ðstatÞ 0.61ðsystÞ 193.3 (21,45) 32.3 33.22.2ðstatÞ 3.2ðsystÞ 0.7ðtheorÞ 3.5< y <4.0 5.770.76ðstatÞ 0.58ðsystÞ 208.9 (21,27) 24.1 27.63.6ðstatÞ 2.8ðsystÞ 0.6ðtheorÞ 3.0< y <3.5 6.710.60ðstatÞ 0.55ðsystÞ 193.3 (27,35) 30.9 34.73.1ðstatÞ 2.9ðsystÞ 0.7ðtheorÞ 2.5< y <3.0 6.831.0ðstatÞ 0.75ðsystÞ 177.6 (35,45) 39.6 38.55.6ðstatÞ 4.2ðsystÞ 0.8ðtheorÞ

−3.6< y <−2.6 2.460.31ðstatÞ^þ0.24_−0.28ðsystÞ 8.66 (577,952) 706 28436ðstatÞ^þ27₋₃₂ðsystÞ 26ðtheorÞ

(GeV)

γp

W

102 10³

+p) (nb)ψ J/+p →γ(σ

10 102

103

ALICE (p-Pb) ALICE (Pb-p)

Power law fit to ALICE data H1

ZEUS

JMRT LO JMRT NLO b-Sat (eikonalized) b-Sat (1-Pomeron)

STARLIGHT parameterization

FIG. 3 (color online). Exclusive J=ψ photoproduction cross section off protons measured by ALICE and compared to HERA data. Comparisons toSTARLIGHT, JMRT, and the b-SAT models are shown. The power law fit to ALICE data is also shown.

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the extraction of the underlying σðW_γpÞ strongly model dependent. Moreover, in contrast with p-Pb collisions, there is a large uncertainty in the hadronic survival probability in pp collisions, as well as an unknown contribution from production through Odderon-Pomeron fusion[11,23]. For eachdσ=dymeasurement, they reported a Wþ and a W− solution. These coupled solutions are shown in Fig.4, together with the power law fit to ALICE measurements. Despite these ambiguities and assumptions the LHCb solutions turned out to be compatible with the power law dependence extracted from our data.

In summary, we have made the first measurement of exclusive J=ψ photoproduction off protons inp-Pb collisions at the LHC. Our data are compatible with a power law dependence of σðW_γpÞ up to about 700 GeV in W_γp, corresponding to x∼2×10⁻⁵. A natural explanation is that no change in the behavior of the gluon PDF in the proton is observed between HERA and LHC energies.

The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: State Committee of Science, World Federation of Scientists (WFS) and Swiss Fonds Kidagan, Armenia, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (FINEP), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); National Natural Science Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) and the Ministry of Science and Technology of China (MSTC); Ministry of Education and Youth of the

Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation and the Danish National Research Foundation; The European Research Council under the European Community’s Seventh Framework Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS-IN2P3, the

“Region Pays de Loire,” “Region Alsace,” “Region Auvergne,” and CEA, France; German BMBF and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece;

Hungarian OTKA and National Office for Research and Technology (NKTH); Department of Atomic Energy and Department of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi,” Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF); CONACYT, DGAPA, México, ALFA-EC and the EPLANET Program (European Particle Physics Latin American Network) Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); Polish Ministry of Science and Higher Education; National Science Centre, Poland; Ministry of National Education/

Institute for Atomic Physics and CNCS-UEFISCDI - Romania; Ministry of Education and Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Research; Ministry of Education of Slovakia;

Department of Science and Technology, South Africa;

CIEMAT, EELA, Ministerio de Economía y Competitividad (MINECO) of Spain, Xunta de Galicia (Consellería de Educación), CEADEN, Cubaenergía, Cuba, and IAEA (International Atomic Energy Agency);

Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council (STFC); The United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio; Ministry of Science, Education and Sports of Croatia and Unity through Knowledge Fund, Croatia.

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G. Scioli,²⁶E. Scomparin,¹⁰⁷ R. Scott,¹²⁰G. Segato,²⁸J. E. Seger,⁸²Y. Sekiguchi,¹²¹ I. Selyuzhenkov,⁹³ J. Seo,⁹² E. Serradilla,^10,60A. Sevcenco,⁵⁸ A. Shabetai,¹⁰⁹G. Shabratova,⁶²R. Shahoyan,³⁴A. Shangaraev,¹⁰⁸N. Sharma,¹²⁰ S. Sharma,⁸⁶K. Shigaki,⁴³K. Shtejer,^25,9Y. Sibiriak,⁹⁶S. Siddhanta,¹⁰²T. Siemiarczuk,⁷³D. Silvermyr,⁸⁰C. Silvestre,⁶⁷ G. Simatovic,¹²³R. Singaraju,¹²⁶R. Singh,⁸⁶S. Singha,^126,75V. Singhal,¹²⁶B. C. Sinha,¹²⁶T. Sinha,⁹⁷B. Sitar,³⁶M. Sitta,³⁰ T. B. Skaali,²¹K. Skjerdal,¹⁷M. Slupecki,¹¹⁸N. Smirnov,¹³¹ R. J. M. Snellings,⁵³C. Søgaard,³²R. Soltz,⁷¹J. Song,⁹²

M. Song,¹³²F. Soramel,²⁸S. Sorensen,¹²⁰ M. Spacek,³⁷E. Spiriti,⁶⁸I. Sputowska,¹¹² M. Spyropoulou-Stassinaki,⁸⁴ B. K. Srivastava,⁹¹J. Stachel,⁸⁹I. Stan,⁵⁸G. Stefanek,⁷³M. Steinpreis,¹⁹E. Stenlund,³²G. Steyn,⁶¹ J. H. Stiller,⁸⁹ D. Stocco,¹⁰⁹M. Stolpovskiy,¹⁰⁸P. Strmen,³⁶A. A. P. Suaide,¹¹⁵T. Sugitate,⁴³C. Suire,⁴⁷M. Suleymanov,¹⁵R. Sultanov,⁵⁴

M. Šumbera,⁷⁹T. Susa,⁹⁴T. J. M. Symons,⁷⁰A. Szabo,³⁶ A. Szanto de Toledo,¹¹⁵I. Szarka,³⁶A. Szczepankiewicz,³⁴ M. Szymanski,¹²⁸J. Takahashi,¹¹⁶M. A. Tangaro,³¹J. D. Tapia Takaki,^47,hA. Tarantola Peloni,⁴⁹A. Tarazona Martinez,³⁴

M. G. Tarzila,⁷⁴A. Tauro,³⁴G. Tejeda Muñoz,² A. Telesca,³⁴C. Terrevoli,²³J. Thäder,⁹³D. Thomas,⁵³R. Tieulent,¹²⁴ A. R. Timmins,¹¹⁷A. Toia,^49,104 V. Trubnikov,³ W. H. Trzaska,¹¹⁸ T. Tsuji,¹²¹A. Tumkin,⁹⁵R. Turrisi,¹⁰⁴T. S. Tveter,²¹ K. Ullaland,¹⁷A. Uras,¹²⁴G. L. Usai,²³M. Vajzer,⁷⁹M. Vala,^55,62L. Valencia Palomo,⁶⁶S. Vallero,^25,89P. Vande Vyvre,³⁴ J. Van Der Maarel,⁵³J. W. Van Hoorne,³⁴M. van Leeuwen,⁵³A. Vargas,² M. Vargyas,¹¹⁸ R. Varma,⁴⁴M. Vasileiou,⁸⁴ A. Vasiliev,⁹⁶V. Vechernin,¹²⁵M. Veldhoen,⁵³A. Velure,¹⁷M. Venaruzzo,^24,69E. Vercellin,²⁵S. Vergara Limón,²R. Vernet,⁸

M. Verweij,¹²⁹ L. Vickovic,¹¹¹G. Viesti,²⁸J. Viinikainen,¹¹⁸ Z. Vilakazi,⁶¹O. Villalobos Baillie,⁹⁸A. Vinogradov,⁹⁶ L. Vinogradov,¹²⁵ Y. Vinogradov,⁹⁵T. Virgili,²⁹ Y. P. Viyogi,¹²⁶A. Vodopyanov,⁶²M. A. Völkl,⁸⁹K. Voloshin,⁵⁴ S. A. Voloshin,¹²⁹G. Volpe,³⁴B. von Haller,³⁴I. Vorobyev,¹²⁵D. Vranic,^93,34J. Vrláková,³⁸B. Vulpescu,⁶⁶A. Vyushin,⁹⁵

B. Wagner,¹⁷ J. Wagner,⁹³V. Wagner,³⁷M. Wang,^7,109Y. Wang,⁸⁹D. Watanabe,¹²²M. Weber,^34,117J. P. Wessels,⁵⁰ U. Westerhoff,⁵⁰J. Wiechula,³³J. Wikne,²¹M. Wilde,⁵⁰G. Wilk,⁷³J. Wilkinson,⁸⁹M. C. S. Williams,¹⁰¹B. Windelband,⁸⁹ M. Winn,⁸⁹C. G. Yaldo,¹²⁹Y. Yamaguchi,¹²¹H. Yang,⁵³P. Yang,⁷S. Yang,¹⁷S. Yano,⁴³S. Yasnopolskiy,⁹⁶J. Yi,⁹²Z. Yin,⁷ I.-K. Yoo,⁹²I. Yushmanov,⁹⁶V. Zaccolo,⁷⁶C. Zach,³⁷A. Zaman,¹⁵C. Zampolli,¹⁰¹S. Zaporozhets,⁶²A. Zarochentsev,¹²⁵ P. Závada,⁵⁶N. Zaviyalov,⁹⁵H. Zbroszczyk,¹²⁸I. S. Zgura,⁵⁸M. Zhalov,⁸¹H. Zhang,⁷X. Zhang,^7,70Y. Zhang,⁷C. Zhao,²¹ N. Zhigareva,⁵⁴D. Zhou,⁷F. Zhou,⁷Y. Zhou,⁵³Zhuo Zhou,¹⁷H. Zhu,⁷J. Zhu,⁷X. Zhu,⁷A. Zichichi,^12,26A. Zimmermann,⁸⁹

M. B. Zimmermann,^50,34G. Zinovjev,³ Y. Zoccarato^1,24 and M. Zyzak⁴⁹

(ALICE Collaboration)